Threats, biodiversity drivers and restoration in temperate floodplain forests related to spatial scales

Flooding drives plant diversity-biomass relationships in riparian zones of the three Gorges reservoir area

Understanding the mechanisms underlying diversity-biomass relationships (DBRs) under hydrological stress is particularly valuable for predicting how increased flooding will impact biodiversity and ecosystem functions. However, it remains unclear how dam-induced extreme flooding influences the biodiversity effect on aboveground biomass (AGB) and belowground biomass (BGB) in riparian ecosystems, and how plant traits and edaphic factors mediate this influence. Here, we surveyed plant diversity, plant traits, AGB, and BGB in 321 quadrats at 107 transects along the hydrological gradients in the Three Gorges riparian zones. We employed structure equation models (SEM) to elucidate the causal mechanisms among flooding, plant diversity, functional traits, soil nutrients, and biomass. We found that AGB was positively associated with the community-weighted means of plant height, leaf length, and leaf dry matter content, as well as the functional dispersion of leaf length, suggesting the simultaneous occurrence of positive selection and complementarity effects. Conversely, BGB was positively associated with species richness, Shannon diversity, and Simpson diversity, highlighting the importance of species complementarity for BGB. Moreover, our SEM explained 17%-47% of AGB and 18%-33% of BGB, with greater explanatory power at the transect scale than the quadrat scale. At both scales, increased flooding stresses directly reduced AGB and BGB, while plant traits (e.g., plant height and leaf dry matter content) and soil nutrients primarily mediated the effects of flooding on AGB and BGB, respectively. Our findings reveal distinct mechanisms driving DBRs for AGB and BGB, highlighting that DBRs are enhanced in habitats with greater heterogeneity shaped by dam-induced hydrological changes.

Ash dieback and hydrology affect tree growth patterns under climate change in European floodplain forests

Floodplain forests are currently undergoing substantial reorganization processes due to the combined effects of management-induced altered hydrological conditions, climate change and novel invasive pathogens. Nowadays, the ash dieback is one of the most concerning diseases affecting European floodplain forests, causing substantial tree mortality and threatening the loss of the dominant key tree species of the hardwood floodplain forest, Fraxinus excelsior. Understanding how the increased light availability caused by pathogen-driven mortality in combination with altered hydrological conditions and climate change affects growth responses in a diverse forest community is of crucial importance for conservation efforts. Thus, we examined growth of the main tree species in response to ash dieback and how it depended on altered hydrological conditions under novel climatic conditions for the lower and upper canopy in the floodplain forest of Leipzig, Germany. Our study period encompassed the consecutive drought years from 2018 to 2020. We found that tree growth responded mostly positively to increased light availability, but only on moist sites, while tree growth largely declined on dry sites, suggesting that water availability is a critical factor for tree species to be able to benefit from increased light availability due to canopy disturbances caused by ash dieback. This hydrological effect was species-specific in the lower canopy but not in the upper canopy. While, in the lower canopy, some species such as the competitive shade-tolerant but flood-intolerant Acer pseudoplatanus and Acer platanoides benefited from ash dieback on moist sites, others were less affected or suffered disproportionally, indicating that floodplain forests might turn into a novel ecosystem dominated by competitive Acer species, which may have detrimental effects on ecosystem functioning. Our results give hints on floodplain forests of the future and have important implications for conservation measures, suggesting that a substantial revitalization of natural hydrological dynamics is important to maintain a tree composition that resembles the existing one and thus sustain their conservation status.

Drivers of the taxonomic and functional structuring of aquatic and terrestrial floodplain bird communities

Context

There has been a limited amount of research which comparatively examines the local and landscape scale ecological determinants of the community structure of both riparian and aquatic bird communities in floodplain ecosystems.

Objectives

Here, we quantified the contribution of local habitat structure, land cover and spatial configuration of the sampling sites to the taxonomical and functional structuring of aquatic and terrestrial bird communities in a relatively intact floodplain of the river Danube, Hungary.

Methods

We used the relative abundance of species and foraging guilds as response variables in partial redundancy analyses to determine the relative importance of each variable group.

Results

Local-scale characteristics of the water bodies proved to be less influential than land cover and spatial variables both for aquatic and terrestrial birds and both for taxonomic and foraging guild structures. Purely spatial variables were important determinants, besides purely environmental and the shared proportion of variation explained by environmental and spatial variables. The predictability of community structuring generally increased towards the lowest land cover measurement scales (i.e., 500, 250 or 125 m radius buffers). Different land cover types contributed at each scale, and their importance depended on aquatic vs terrestrial communities.

Conclusions

These results indicate the relatively strong response of floodplain bird communities to land cover and spatial configuration. They also suggest that dispersal dynamics and mass-effect mechanisms are critically important for understanding the structuring of floodplain bird communities, and should therefore be considered by conservation management strategies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10980-024-01948-3.

Historical disconnection from floodplain alters riparian forest composition, tree growth and deadwood amount

Riparian forests are among the most dynamic but threatened terrestrial ecosystems. Their dynamism and conservation depend on historical changes in river geomorphology, which can be evaluated through changes in channel sinuosity. However, we lack long-term assessments on sinuosity and how they impact riparian forest composition, tree growth and deadwood amount. To fill this research gap, we reconstructed river sinuosity in 14 sites across the middle Ebro basin, north-eastern Spain, using historical aerial photographs taken in 1927, 1956, 1998-2003 and 2014-2015. Relationships between sinuosity, stand composition and deadwood amount and decay degree were calculated. We also reconstructed radial growth of the major tree species (Populus alba, Populus nigra, Fraxinus angustifolia, Salix alba and Ulmus minor) in two sites to evaluate how coupled it was with changes in river flow after dam building. From 1927 to 2015, sinuosity decreased passing from 1.39 to 1.20. The river dynamics were altered in the 1950s and 1960s after dam and dyke building. Sites with high sinuosity values in 1956 corresponded to mature stands with large P. nigra individuals. Sinuosity was negatively related to F. angustifolia (rs = -0.83, p < 0.001) and P. alba (rs = -0.64, p = 0.02) abundance, whereas sites dominated by P. alba and U. minor presented abundant decayed deadwood. A loss of sinuosity and a contraction of the riverbank gradient increased disconnection of active channel from floodplain, with a mixing of more (e.g., P. nigra) and less phreatophytic species (e.g., U. minor). River flow diversion reduced growth and increased the tree-to-tree P. alba growth coherence. Hydrological droughts contributed to growth decline and dieback of U. minor, which is sensitive to spring river flow. Conservation and restoration of riparian forests must consider historical changes in river geomorphology related to human activities.

Classification of Wetland Forests and Scrub in the Western Balkans

Wetland forests and scrub (WFS) are conditioned by the strong impact of water. They consist of various vegetation types, depending on many factors such as type and duration of flooding, water table level and its fluctuation, river current strength, substrate ability to retain water, etc. WFS vegetation has been insufficiently studied in the Balkan Peninsula, especially in Bosnia and Herzegovina. By means of numerical classification, we aimed to classify Western Balkans WFS at the alliance level, and to identify the main underlying ecological gradients driving the variation in species composition. The dataset containing all published and available unpublished relevés from Slovenia, Croatia and Bosnia and Herzegovina was first classified using the EuroVegChecklist Expert System in Juice software in order to assign the corresponding class to each of the relevés. Relevés were subsequently analyzed within each of the four WFS classes (Alno glutinosae-Populetea albae, Salicetea purpureae, Alnetea glutinosae and Franguletea). Cluster analysis resulted in eight alliances, Salicion albae, Salicion triandrae, Salicion eleagno-daphnoidis, Alno-Quercion, Alnion incanae, Alnion glutinosae, Betulion pubescentis and Salicion cinereae, while one cluster could not be assigned with certainty. Edafic factors were found to be the most important factors determining the floristic composition and syntaxa differentiation of WFS in the study area.